Abstract Transient absorption spectra and triplet decay rates k d of thioxanthone (TX) and 2,4-diisopentylthioxanthone (DITX) in solid polymer matrices were measured using an Nd:YAG laser flash photolysis technique. In a polystyrene matrix, the triplet decay rates of TX and DITX are not dependent on their concentration, indicating that the self-quenching of TX and DITX is not effective in a polymer matrix. In a poly(styrene-co-vinylbenzyl azide) (AZMS) matrix, the triplet decay rates of TX and DITX increase as the number of azido groups in the AZMS polymer increases. The energy transfer rate k et to the azido group in the AZMS polymer matrix is 1.5×10 6 M −1 s −1 for TX and 1.4×10 6 M −1 s −1 for DITX. The k d/ k et values of TX and DITX in the AZMS polymer matrix are 20–30 times smaller than the values previously obtained from the measurement of the amount of nitrogen evolved under steady state conditions. The disagreement between the two results strongly indicates that the autocatalytic chain decomposition reaction of the azido group takes place in the AZMS polymer matrix.